Polyphosphate (poly (P)) is a linear polymer of hundreds of orthophosphate (Pi) residues linked by high-energy phosphoanhydride bonds. Many lines of evidence indicate that poly (P) plays a critical role in regulatory responses to stresses and nutritional deficiencies. Polyphosphate kinase (PPK) is an enzyme responsible for the reversible synthesis of poly (P) from ATP. It has been shown recently that PPK is essential for biofilm development, quorum sensing, motility and release of virulence factors of many pathogenic microorganisms, including P. aeruginosa. Knockout of the PPK gene leads to the loss of viability of the pathogenic microorganism. Thus PPK specific inhibitors could become a novel family of antibiotics, which can be very useful to overcome antibiotic resistance in cystic fibrosis and other immunodeficient patients, and could play a role in combating bioterrorism. PPK is a membrane-associated enzyme that does not have apparent sequence homology with other proteins. No three-dimensional structural information is available for the PPK family. We have recently purified and crystallized the full-length E. coli PPK, and the initial phases for structure determination by x-ray crystallography have been obtained. The main goal of this proposal is the determination of crystal structures of PPK and its complexes with various substrates and reaction intermediate mimics. Our structural studies will aid in determining the PPK catalytic mechanism and provide insights into drug design. [unreadable] [unreadable]